Check a concentration change with a sanity ratio

Check a concentration change with a sanity ratio

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When you change your reconstitution volume, every dose volume changes with it, and this is the moment where a decimal point error does the most damage. There is a five second check that catches almost all of them, and it does not require redoing the arithmetic.

Take the ratio. If you halved the diluent volume, the concentration doubled and every dose volume must halve. If you went from 2 mL to 3 mL, concentration dropped by a third and volumes go up by half. Compare your new dose volume against the old one and confirm the direction and rough size of the change before you trust the number.

The reason this works is that it is a completely different route to the answer. Your original calculation went through concentration. The ratio check goes through proportion. Two independent methods agreeing is meaningful; recomputing the same division twice is not, because you will make the same mistake twice.

The specific error it catches is the one where the answer is out by a factor of ten and looks plausible in isolation. A dose volume of 0.4 mL and one of 0.04 mL are both perfectly ordinary numbers on their own. Only against the previous vial does one of them become obviously wrong.

Write the old and new concentrations next to each other when you make the change. In a Peptide Tracker & Calculator the previous vial's numbers are already stored, so the comparison is on screen rather than in your head, and the check costs nothing except the discipline of actually looking.

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